A test version of the NHS's coronavirus contact-tracing app has been published to Apple and Google's app stores.

Council staff and healthcare workers on the Isle of Wight will be invited to install it on Tuesday, ahead of a wider roll-out on the island on Thursday.

Project chiefs have said their so-called "centralised" approach gives them advantages over a rival scheme advocated by the US tech giants and some privacy experts.

But fresh concerns have been raised.

The Information Commissioner's Office has declared that "as a general rule, a decentralised approach" would better follow its principle that organisations should minimise the amount of personal data they collect.

The House of Commons' Human Rights Select Committee also discussed fears about plans to extend the app to record location data.

"There is an inherent risk that if you create a system that can be added to incrementally, you could do so in a way that is very privacy invasive," cautioned law professor Orla Lynskey.

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Media captionMatt Hancock said Isle of Wight residents using the app "will be saving lives"

"By downloading the app, you're protecting your own health, you're protecting the health of your loved ones, and the health of the community."

Wireless signals

The NHS Covid-19 app is intended to supplement medical tests and contact-tracing interviews carried out by humans, in order to prevent a resurgence of Covid-19 when lockdown measures are eased.

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Media captionWATCH: What is contact tracing and how does it work?

It works by using Bluetooth signals to detect when two people's smartphones are close to each other. If one person later registers themselves as being infected, an alert can be sent to others judged to be at high risk of contagion. This might be based on the fact they were exposed to the same person for a long period of time or that there had been multiple instances of them being in the vicinity of different people.

The trial on the Isle of Wight will help NHSX test how well the system works in practice, as well as judge how willing a population is to install and use the software. It follows a smaller experiment on an RAF base.

Although the app is live, it is effectively hidden on the iOS and Android marketplaces, and residents will need to follow a set of instructions to install it.

Image copyrightNHSImage caption
Users will be asked to enter the first part of their postcode but not their name or other personal details

While in theory there is nothing to prevent the details being shared and used by others elsewhere, NHSX hopes this will not happen as it could confuse the feedback it receives.

Ahead of the trial, NHSX chief Matthew Gould acknowledged that there would "inevitably be unintended consequences" and that "if we think there is a better way of doing what we need to do, we won't hesitate to change".

But he added that if citizens "want to carry on saving lives, protecting the NHS and get the country back on its feet, then downloading the app is one way they can do that".

More data

NHSX's app will send back details of the logged Bluetooth "handshakes" to a UK-based computer server to do the contact matching, rather carrying out the process on the handsets themselves.

Apple, Google and hundreds of privacy advocates have raised concerns that this risks hackers or even the state itself being able to re-identify anonymised users, and thus learn details about their social circles.

The Isle of Wight's Green Party - which has nine locally-elected councillors - has also expressed its doubts.

"The Isle of Wight has a significantly older and more vulnerable population [and] the island's one hospital could be overwhelmed if... people feel they do no need to stick to lockdown measures due to the rolling out of this app," it said.

But the government's coordinator for testing said the island was "well-equipped" to cope.

"It's quite a large population and there is a benefit in the fact that travel on and off the island is relatively restricted - the ferries are there, but they're running relatively infrequently," added Public Health England's Prof John Newton.

"So it is an ideal place to look at the epidemiology and see the impact."